1. Field of the Invention
The present invention relates to a starting device of an automatic transmission for a vehicle, in particular, to a starting device which is suitable for an automatic transmission mounting a belt driven continuously variable transmission, in particular, a starting device having a hydraulic coupling (including torque converter) and a centrifugal type lock-up clutch for an automatic transmission of a vehicle.
2. Description of the Prior Art
Conventionally, with regard to an automatic transmission mounting a belt driven continuously variable transmission (CVT), as shown in the Japanese Laid Open Patent No. Sho-57-140956, an automatic transmission having a hydraulic coupling such as a torque converter and a fluid coupling, and a centrifugal type lock-up clutch have been proposed.
In general, a centrifugal type frictional clutch used for a lock-up clutch of a hydraulic coupling is composed of a weight, a retractor spring whose ends are supported by a pin and providing the weight with a pressing force opposing the centrifugal force, a shoe which can touch a case of the hydraulic coupling, and a main spring which is situated between the shoe and the retractor spring. The characteristic of the centrifugal frictional clutch is shown in FIG. 14.
Namely, in the case that rotation of the output side of the hydraulic coupling is below a certain rotational speed, a slit is made between the case and the shoe based on the retractor spring, and the centrifugal frictional clutch is in a disconnected condition (i.e., the no motion area A of FIG. 14). In this condition, torque from the engine is transmitted to an automatic transmission through the hydraulic coupling, so a vehicle starts smoothly. In the case that rotation of the output side of the hydraulic coupling is over a certain rotational speed, by centrifugal force the weight pushes the retractor spring, the shoe contacts with the case, and then torque transmission begins (i.e., the low speed area B). In the area B, the torque capacity increment ratio of the clutch becomes large because the shoe is pressed by centrifugal force on the weight and furthermore centrifugal force on the shoe itself is added. Due to this, fuel efficiency during low speed running is enhanced. Still further rotation of the output side of the hydraulic coupling increases, the weight contacts with the pin, centrifugal force action on the weight does not act on the shoe, and torque transmission is conducted based on centrifugal force action on the shoe itself (i.e., the medium speed area C). In the area C, the torque capacity increment ratio is relatively low, and torque vibration caused by knocking, etc., and shift shock are absorbed by slip of the clutch (torque limiter function). In the case that rotation of the output side of the hydraulic coupling yet increases, the main spring extends outwardly by centrifugal force, and centrifugal force developed by the main spring itself is added, so the lock-up torque curve in FIG. 14 becomes steep (i.e., the high speed area D). The curve labelled by the letter E in FIG. 12 represents the maximum torque of the engine.
Even in the case of the centrifugal lock-up clutch featuring a torque limiter function, when a high load exists and the rotation of the output side rises, for example at a time of down-shifting, a stick slip of the clutch occurs due to the increase of required clutch capacity and high load along with the rise of rotation, and as a result, smooth acceleration by kick down is hampered, particularly in the high speed area D and in a high speed side of the medium speed area C, shift shock occurs because the clutch capacity is too large.
On the other hand, in the case that the torque capacity of the clutch is set to be small, even under normal running conditions on flat roads, the lock-up clutch may frequently have slip (particularly in the low speed area B and the low speed side of the medium speed area C), and consequently fuel efficiency is lowered.
While a starting device employing a slip clutch whose slip ratio is controlled by microcomputer so that a vehicle can start smoothly has been proposed, a device of this type requires a complicated and intricate control device to detect vehicle load and rate of revolution of the engine. Furthermore, a response delay in the control device inevitably occurs depending on the starting situation, and such delay causes a matching deficiency between the operation of a driver, and consequently, the starting performance is lowered.